You really need to provide a basic child theme, could be used on all the themes as a template. Really should be quick job for you mate. Since most child themes require the proper enqueue of the parent theme etc. Using a plugin isn’t always a solution, nor a preferred one. It’s a bit of a deal breaker.. if we can’t get a functioning child theme, then we can’t build.
It really comes down to level of service and how easy you make it for customers. For 1-2 days of work, you could whip out the child themes and make it easier for everyone. Sending people to WordPress.org tutorials doesn’t achieve that, and nor does being directed to keep loading up more plugins. What if clients uninstall the plugin – then what? Though this one, might keep the files after its deactivated. I haven’t tested either.
OceanWP has a child theme, Hello Elementor has one one, and so do most theme providers. It makes development much easier, and guarantees there won’t be loss if clients offload plugins. I did get a child theme to work, but I ended up having to enqueue the child style.css file as well.